A martensitic stainless steel such as API-13% Cr, which is used as a pipe for an oil well, normally includes a carbon content of about 0.2%, which needs a high yield strength of 80 ksi (552 MPa) or more and a hot workability. Due to a high C and Cr content, an as-rolled stainless steel pipe has an extreme hardness, therefore has a reduced toughness. Consequently, an as rolled conventional martensitic stainless steel pipe might have a crack resulting from a delayed fracture in “the impact-worked portion”, where an impact load or static load was worked before a heat treatment. Accordingly, it is necessary to restrict the piling height in “a rack” and/or the dropping height into a rack of the steel pipes during transportation or storage. Moreover, the stand-by time before a heat treatment after hot-rolling must be shortened.
The above-mentioned restrictions during transportation or storage could result in various disadvantages such as a large stockyard because of the restriction of the piling height and/or the dropping height of pipes, a reduction in working efficiency resulting from the careful handling of the steel pipes without excessive loading impact and a restricted time schedule from hot rolling to a heat treatment in order to finish the heat treatment within a restricted working period.
Japanese Patent Unexamined Publication No. H8-120415 discloses a martensitic stainless steel having a restricted N content. In this patent specification, only the improvement of toughness after a heat treatment is described. However, neither the relationship between the N content and a delayed fracture in the impact-worked portions of an as-rolled steel pipe nor the measures for suppressing such inner surface defects as internal scabs due to poor hot workability resulting from the decreased N content is described. It is not practical to manufacture a seamless steel pipe without any measures to suppress internal scabs.
Japanese Patent Unexamined Publication No. H6-306551 discloses an invention, in which the hydrogen content is restricted to improve the toughness in the heat affected zone by welding of a martensitic stainless steel pipe having low carbon content. Furthermore, Japanese Patent Unexamined Publication No. H5-255734 describes an invention of dehydrogenating a martensitic stainless steel having low carbon content in order to prevent a delayed fracture. These inventions deal with a martensitic stainless steel having low carbon content. However, no description is given regarding the relationship between the hydrogen content and a delayed fracture in the impact-worked portions of an as-rolled martensitic stainless steel pipe containing such high C of about 0.2%.